Optical elements for high efficiency conversion of the monochromatic light emitted by light emitting diodes (LEDs) or lasers may be comprised of photoluminescent materials, such as inorganic phosphors, that at least partially convert the primary light emitted by these devices into a longer wavelength secondary light which can be used to achieve a different single color emission or a white light. A preferred conversion material is cerium-activated yttrium aluminum garnet (YAG:Ce) which is used in combination with a blue-emitting LED to generate white light. In some applications, conversion is achieved with conversion elements comprised of polycrystalline ceramics or single crystal materials. Such solid ceramic converters are preferred for high power applications because of their ability to better dissipate the heat generated by the conversion process. Polycrystalline materials are more preferred because single crystals are generally more expensive and limited in their applicability.
More particularly, single crystals are generally grown by slowly pulling a seeded crystal from a melted liquid with techniques such as the Czochralski (CZ) method. Unfortunately, such growth methods limit the concentration of activator ions that can be incorporated in the host lattice because of (1) the ionic size mismatch between the activator ions and the host lattice ions, and (2) the segregation coefficients of the activator ions. This consequently leads to an upper limit of about 0.33% Ce substitution for Y or Lu in garnet-based phosphors such as YAG:Ce or cerium-activated lutetium aluminum garnet (LuAG:Ce.) CZ grown crystals are also subject to optically heterogeneous structures such as cores, facets around the outer region, and striations which makes them less suitable for certain applications.
Some applications of optical single crystals may also benefit from a concentration gradient of activator ions which is difficult to generate in melt grown crystals. Gradients might be introduced by diffusion but this limits the location of the gradients (e.g., only higher outside where the dopant can be applied for diffusion). Furthermore, single crystals pulled from a melt are generally cylindrical in shape and need extensive machining to make desired shapes and physical features.
In sum, current melt-grown single crystals have limited activator contents, limited placements for activator concentration gradients, limited as-grown shapes, slow melt processing, and optical heterogeneities.